Our Scientists

Understanding how growth is controlled is a major goal of developmental biology. Decades ago, regeneration experiments revealed an intimate relationship between organ patterning and organ growth, but the molecular basis for this relationship has remained elusive. We are engaged in projects whose long-term goal is to define relationships between patterning and growth in developing and regenerating organs. Much of our research takes advantage of the powerful genetic, molecular, and cellular techniques available in Drosophila melanogaster, which facilitate both gene discovery and the analysis of gene function.

Our current research focuses on a novel signaling pathway, the Fat-Hippo pathway, which plays important roles in growth control from Drosophila to humans. We study both the molecular mechanism of Fat-Hippo signaling and its developmental functions.

Mechanism of Fat-Hippo Signaling
The Fat signaling pathway is named for a Drosophila gene, fat, which encodes a cadherin protein that acts as a transmembrane receptor for this pathway. Fat signaling influences both gene expression and planar cell polarity (PCP). Several years ago, we discovered that the influence of Fat signaling on gene expression is effected through an intersection with the Hippo-Warts signaling pathway. Several components of this pathway act as tumor suppressors or oncogenes, both in Drosophila and in mammals. Our studies have analyzed multiple steps of Fat signaling, from regulation of the Fat receptor at the membrane, to the intersection with Warts-Hippo signaling, to the regulation of gene expression through the transcriptional coactivator Yorkie.

Genetic studies in Drosophila identified the four-jointed gene (fj) as a regulator of Fat signaling. More recently, we found that fj encodes a novel protein kinase that phosphorylates specific Ser or Thr residues within cadherin domains of Fat and its transmembrane ligand, Dachsous (Ds). Fj, which functions in the Golgi to phosphorylate Fat and Ds, was the first molecularly identified Golgi kinase. Modification of Fat by Fj promotes Fat-Ds binding, whereas modification of Ds by Fj inhibits Fat-Ds binding. As a consequence of these opposing effects, juxtaposition of cells differing in their Fj levels results in asymmetric Fat-Ds binding, which we think helps to polarize cells.

We have also investigated the nature of Fat receptor activation. discs overgrown (dco) encodes Drosophila CKIε (casein kinase Iε), which we placed into the Fat-Hippo pathway through genetic experiments. More recently, we determined that the cytoplasmic domain of Fat is phosphorylated by Dco. As this Dco-mediated Fat phosphorylation is promoted by Ds, it provides the first biochemical marker of Fat receptor activation. Our evaluation of dco mutants indicated that they affect Fat’s influence on growth and gene expression, but not its influence on PCP, which implies that Fat is activated in distinct ways for Hippo versus PCP signaling.

The dachs gene occupies a central position within the Fat signaling pathway, as dachs influences both the transcriptional and PCP outputs of Fat signaling. dachs encodes a myosin-related protein, and Fat controls the subcellular localization of Dachs protein: absence of Fat allows Dachs to accumulate on the membrane, whereas active Fat forces Dachs off the membrane. These observations imply that Fat signaling is transmitted through its influence on Dachs localization.

Yorkie, a transcription factor of the Fat and Hippo signaling pathways, is negatively regulated by the Warts kinase. Characterization of Warts-dependent phosphorylation of Yorkie in vivo revealed that Warts promotes phosphorylation of Yorkie at multiple sites, and we characterized their contributions to Yorkie regulation. Investigations of Yorkie that couldn't be phosphorylated on these sites led us and others to characterize another type of Yorkie regulation, in which Warts or Expanded can repress Yorkie by directly binding to it.

We are continuing to investigate multiple steps in Fat-Hippo signaling, using a combination of genetic, histological, and biochemical approaches. In parallel with this, we are investigating how the pathway is used and regulated in different developmental contexts.

Developmental Roles of Fat-Hippo Signaling
Many signaling pathways are regulated by ligands expressed in gradients. However, conventionally, a ligand gradient is interpreted according to the intensity of the signal: i.e., higher levels elicit a stronger signal, and lower levels elicit a weaker signal. One remarkable feature of Fat signaling is that rather than receptor activity being governed simply by the amount of ligand, Fat signaling can be influenced by the vector and slope of the Ds and Fj gradients, with the vector influencing PCP and the slope influencing transcription.

The influence of the Fj and Ds gradients on Fat-Hippo signaling was established through experiments in which we demonstrated that juxtaposition of cells that express different levels of fj or ds stimulates expression of Fat-Hippo pathway target genes and cell proliferation, whereas uniform expression of fj and ds in the wing inhibits cell proliferation. We also linked the graded expression of fj and ds to the Decapentaplegic (Dpp) morphogen gradient and showed that the localization and activity of Fat pathway components, and Fat signaling through Dachs, are required for the influence of the Dpp gradient on cell proliferation. We think the ability of cells to respond to the Fj and Ds gradients can be explained by their ability to polarize Fat activity within cells. Dachs protein is normally asymmetrically localized in the developing wing, and manipulations of Ds and Fj expression revealed that this asymmetry is directed by the Ds and Fj gradients.

Studies of Fat-Hippo signaling in Drosophila initially focused on imaginal discs. More recently, we have investigated roles of the pathway in other organs. Neuroepithelial cells are neural progenitor cells that function during early development as symmetrically dividing stem cells. Fat-Hippo signaling controls both the proliferation and the differentiation of optic neuroepithelial cells. Analysis of the differentiation requirement led us to discover that they need to undergo a cell cycle pause to transition from neuroepithelial cells into neuroblasts. Activation of Yorkie impedes this cell cycle pause. The cell cycle pause appears to be needed because it influences regulation of Notch signaling. Our studies identified a mechanism through which the action of multiple signaling pathways is coordinated during neuronal differentiation by their relation to the cell cycle.

The adult midgut has emerged as a Drosophila model for analysis of somatic stem cells. Midgut intestinal stem cells (ISCs) maintain homeostasis, and if the midgut is damaged, then ISC proliferation increases. We have found that this increase is mediated by the Hippo pathway, but in a nonautonomous fashion. Yorkie is activated in differentiated cells in response to tissue damage. Activation of Yorkie promotes expression of cytokines, which increase proliferation of nearby ISCs through the Jak-Stat pathway. We are also exploring the role and regulation of the pathway in regeneration and response to tissue damage in other organs.

Homologs of many genes in Fat and Hippo signaling are conserved in mammals, but it was not clear whether mammals have a Fat signaling pathway equivalent to that in Drosophila, nor what its roles were. To investigate this, we created a mutation in a murine ds homolog, Dchs1, and have characterized it, together with mutations in a murine fat homolog, Fat4. Our analysis thus far has identified novel requirements for Dchs1-Fat4 signaling in multiple organs.

This research is also supported by a grant from the National Institutes of Health.

As of September 10, 2010